An Artificial Polyacrylonitrile Coating Layer Confining Zinc Dendrite Growth for Highly Reversible Aqueous Zinc‐Based Batteries

Abstract Aqueous rechargeable zinc‐metal‐based batteries are an attractive alternative to lithium‐ion batteries for grid‐scale energy‐storage systems because of their high specific capacity, low cost, eco‐friendliness, and nonflammability. However, uncontrollable zinc dendrite growth limits the cycle life by piercing the separator, resulting in low zinc utilization in both alkaline and mild/neutral electrolytes. Herein, a polyacrylonitrile coating layer on a zinc anode produced by a simple drop coating approach to address the dendrite issue is reported. The coating layer not only improves the hydrophilicity of the zinc anode but also regulates zinc‐ion transport, consequently facilitating the uniform deposition of zinc ions to avoid dendrite formation. A symmetrical cell with the polymer‐coating‐layer‐modified Zn anode displays dendrite‐free plating/stripping with a long cycle lifespan (>1100 h), much better than that of the bare Zn anode. The modified zinc anode coupled with a Mn‐doped V2O5 cathode forms a stable rechargeable full battery. This method is a facile and feasible way to solve the zinc dendrite problem for rechargeable aqueous zinc‐metal batteries, providing a solid basis for application of aqueous rechargeable Zn batteries.


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Synthesis of Mn-doped V 2 O 5 (MnVO). 0.37 g V 2 O 5 ordered from Sigma-Aldrich (> 99.6 %) was dissolved in 50 mL distilled water with 2 mL H 2 O 2 (local vendor, 30%) added. 0.17 g MnSO 4 •H 2 O (Sigma-Aldrich, ≥ 98%) was dissolved in 30 mL distilled water. The two solutions were then transferred to a 100 mL Teflon container in a stainless-steel autoclave that was heated to 120 °C at a rate of 5 °C min -1 and kept at 120 °C for 10 h. Brick-red precipitates were obtained by centrifugation and washed several times with distilled water followed by absolute ethanol. After drying in a vacuum oven at 80 °C for 12 h, the product's color changed to greenish.
Preparation of positive electrode. 30 mg colloidal dispersion containing 60 wt.% PTFE binder in distilled water was dispersed into 5 mL absolute ethanol by sonication. Then, 126 mg MnVO and 36 mg carbon black were transferred into the above prepared PTFE binder dispersion solution, which was then sonicated for 30 min to get a cathode slurry. The slurry was dried under an infrared lamp and then rolled into one piece, which was cut to 0.5 mm × 0.5 mm squares and then placed onto stainless-steel mesh. The area mass loading for the cathode electrode was ~4 mg cm -2 .
Contact angle measurements. The contact angle values of bare Zn, PAN coated Zn disk without any salt (PAN@Zn), and the PANZ@Zn electrode were measured on an optical contact angle goniometer (KINO INDUSTRY Co., Ltd., Model #SL200KS, USA). The contact angles ranging from 1° to 180° were tested with 4 μL of electrolyte at room temperature for 0 min, 4 min, 8 min, 12 min, 16 min, and 20 min, successively.
Ionic conductivity. Ionic conductivity of PANZ layer was measured by sandwiching the PANZ film wetted by 2 M Zn(TfO) 2 in two blocking electrodes (stainless steel sheets) for the electrochemical impedance spectroscopy (EIS) measurement with an amplitude of 10 mV over a frequency from 100kHz to 0.1 Hz (). The ionic conductivity was calculated by the following equation: where l (mm) represents the thickness of PANZ layer and S (cm 2 ) is the area of the contact surface between one of the stainless sheets and the PANZ layer. R (Ω)is the intercept with real axis.
Material Characterization. The electrodes' morphology images were obtained using a desktop scanning electron microscope (SEM, Phenom). The X-ray diffraction (XRD) patterns of electrodes and MnVO were recorded using a Rigaku Smart Lab TM diffractometer using Cu Kα radiation.
Galvanostatic charge/discharge cycling was performed on a Land (CT2001A Wuhan, China) battery-testing system. Symmetric cells were assembled using zinc disks with a diameter of 10 mm and thickness of 20 μm in ambient atmosphere to evaluate the striping/plating performance of the PAN@Zn, PANZ@Zn, and bare zinc anodes. The long-term cycling performance and cycling stability of the zinc anodes were measured at a current density of 1mA cm -2 with fixed capacity of 1 mAh cm -2 .